1. Field of the Invention
The present invention relates to an apparatus for the coding/decoding of image signals. The invention relates to an apparatus making it possible to process in real time sequential information signals, such as video signals, with a view to compressing the volume of the data necessary for representing the corresponding image, so as to transmit it with a minimum bit rate using a transmission network.
The apparatus according to the invention is more particularly usable in television, picture conference or picturephones. It is also usable for slow variation images as in remote monitoring.
2. Discussion of the Background
With respect to the image compression methods, reference can e.g. be made to the article which was published in Echo des Recherches, No. 126, fourth quarter 1986 and entitled "L'image numerique et le codage". Reference can also be made to EP-A2-84 4270, EP-A2-123 456 and U.S. Pat. No. 4,185,188, which describe the different image coding systems.
In these image coding systems and in general terms in recent image coding systems maximum use is made of the time redundancy of the image signal, i.e. for coding an image account is taken of the preceding image.
It is pointed out that a first known coding type, called the interimage coding procedure consists of comparing the image to be coded with the preceding image and of only transmitting after coding information relating to that part of the image which is moving.
Another known coding type, called coding by movement estimates, consists of anticipating the movement in the image by estimating an image in view of the preceding image and of only transmitting information relating to the difference between said estimated image and the image effectively received.
Conventionally the image coding system comprises a transformation means for applying a transformation operator to said image. This transformation operation translates the image from the spatial field into the frequency field. The transformation operation can precede or follow the interimage coding or movement estimation coding applied to the image.
In practice an image is subdivided into a plurality of blocks before being processed, either by interimage or movement estimation coding, or by the transformation operator, which makes it possible to better define the parts of the image which are modified between two successive images and therefore to reduce the rate transmitted on the transmission network, because only those modified parts are coded and transmitted.
Moreover, in the case of coding by estimating the movement to be transmitted for each block or movement, said transmission consists of a displacement vector indicating the displacement of the block between the preceding image and the existing image. It is normal to break down the images into identical blocks of standard size 8.times.8 pixels. 16.times.16 pixels and sometimes 32.times.32 pixels. The larger the blocks, the better the compression factor, but the compression gain is lower on passing from 16 to 32 pixels and on passing from 8 to 16 pixels.
As can be seen in FIG. 1, a conventional coder-decoder comprises a coding channel OODi, DOODi for reducing the speed of an image i to be transmitted on the network TR and a decoding channel DOODj for receiving and decoding the signal corresponding to a remote image j transmitted by said network TR.